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Linguistic and Cultural Adaptation and Validation of the Vocal Fatigue Index into Polish

Open AccessPublished:October 08, 2022DOI:https://doi.org/10.1016/j.jvoice.2022.09.012

      Summary

      Objective

      The purpose of the present study was the cross-cultural and linguistic adaptation and validation of the Vocal Fatigue Index (VFI) in the Polish language.

      Methods

      The Vocal Fatigue Index was adapted into Polish and was administered to 323 subjects (216 with voice disorders, and 107 without voice disorders). One hundred and four participants with voice disorders completed the VFI a second time to evaluate test-retest reliability. VFI was assessed for internal consistency, test-retest reliability, discriminant validity, criterion validity, and sensitivity and specificity.

      Results

      The Polish VFI exhibited satisfactory psychometric properties. A high Cronbach's alpha coefficient for the three factors (α=0.971 for Factor 1, α=0.929 for Factor 2, α=0.930 for Factor 3) was obtained. Significantly higher scores were revealed in subjects with voice disorders (48.88±11.73 [mean±SD]) in comparison with vocally healthy subjects (12.25±6.75 [mean±SD]) in the total VFI score as well as in the three factors (all P-values < 0.01). The best cut-off for VFI Total Score was set at a 32.5 score with sensitivity of 0.931 and specificity of 0.991.

      Conclusions

      The translated version of the VFI is a valid and reliable tool for identifying vocal fatigue symptoms in the Polish-speaking population.

      Key Words

      INTRODUCTION

      Vocal fatigue (VF) is a complex multifaceted clinical phenomenon and one of the most common complaints of individuals presenting with voice disorders.
      • Mansouri Y
      • Naderifar E
      • Hajiyakhchali A
      • et al.
      The dysphonia severity index as a mediator for the relationship between the vocal fatigue index and the voice-related quality of life among elementary teachers with voice complaint.
      Due to the lack of a universally accepted definition of VF, it has been conceptualized in literature in a variety of ways.
      • Welham NV
      • Maclagan MA
      Vocal fatigue: current knowledge and future directions.
      ,
      • Imaezue GC
      Treatment of vocal fatigue in teachers.
      McCabe and Titze (2002) described vocal fatigue as ”a debilitating condition affecting speakers who place demands on their voice that exceed their vocal capabilities, characterized by a progressive increase in phonatory effort and loss of phonatory abilities.”
      • McCabe DJ
      • Titze IR
      Chant therapy for treating vocal fatigue among public school teachers: a preliminary study.
      In a review of vocal fatigue, Welham and Maclagan define vocal fatigue as “negative vocal adaptation that occurs as a consequence of prolonged voice use.” This negative vocal adaptation is described as “a perceptual, acoustic, or physiologic concept, indicating undesirable or unexpected changes in the functional status of the laryngeal mechanism”.
      • Welham NV
      • Maclagan MA
      Vocal fatigue: current knowledge and future directions.
      A further definition provided by Solomon, defines vocal fatigue as ”the self-report of an increased sense of effort with prolonged phonation, regardless of the presence of observable or measurable deterioration in phonatory function.”
      • Solomon NP
      Vocal fatigue and its relation to vocal hyperfunction.
      On the contrary, Vilkmann (2004) used the term vocal fatigue in reference to the self-perceived status and negative sensations about voicing.
      • Vilkman E
      Occupational safety and health aspects of voice and speech professions.
      Additionally, vocal fatigue is defined as a feeling of localized tiredness and a weak voice after a period of vocal use that involves a set of symptoms during or after speech.
      • Nanjundeswaran C
      • Jacobson BH
      • Gartner-Schmidt J
      • et al.
      Vocal Fatigue Index (VFI): development and validation.
      ,
      • Contreras-Regatero S
      • Vila-Rovira J
      • Verdejo C
      Validity and reliability of spanish version of two questionnaires of vocal fatigue in female teachers.
      Overall, vocal fatigue has been characterized as a consequence of voice use, or a self-perceived set of symptoms with changes in sensation and voice. A most recent consensus definition provided by Hunter et al., (2019) defines vocal fatigue as “a quantifiable decline in function (performance or perceptual) that influences vocal task performance and is individual specific.”
      • Hunter EJ
      • Titze IR
      Quantifying vocal fatigue recovery: Dynamic vocal recovery trajectories after a vocal loading exercise.
      Commonly reported vocal fatigue (VF) symptoms include a sense of discomfort (soreness, pain, and tightness) in the laryngeal area, increased voicing effort, difficulty in voice projection, reduced pitch range, altered voice quality, loss of vocal flexibility, hoarseness, and increased throat clearing.
      • Mansouri Y
      • Naderifar E
      • Hajiyakhchali A
      • et al.
      The dysphonia severity index as a mediator for the relationship between the vocal fatigue index and the voice-related quality of life among elementary teachers with voice complaint.
      ,
      • Yiu EML
      • Chan RMM
      Effect of hydration and vocal rest on the vocal fatigue in amateur karaoke singers.
      Symptoms of VF may be experienced by any individual during their lifetime, however, professional voice users in occupational settings tend to experience this symptom more frequently. Vocal fatigue may lead to an increased vocal effort, and decreased speaking endurance and ultimately may result in voice disorders, such as vocal hyperfunction or vocal nodules.
      • Lei Z
      • Fasanella L
      • Martignetti L
      • et al.
      Investigation of vocal fatigue using a dose-based vocal loading task.
      Vocal fatigue symptoms are often considered the first early indicators of developing voice disorders.
      • Boominathan P
      • Rajendran A
      • Nagarajan R
      • et al.
      Vocal abuse and vocal hygiene practices among different level professional voice users in India: a survey.
      ,
      • Athira UR
      • Devadas U
      Adaptation and Validation of Vocal Fatigue Index (VFI) to Malayalam Language.
      Given that these symptoms usually subside with rest, the individuals experiencing them do not immediately seek help. However, their presence can negatively affect social and occupational functioning, decreasing the overall quality of life.
      • McCabe DJ
      • Titze IR
      Chant therapy for treating vocal fatigue among public school teachers: a preliminary study.
      Identifying the symptoms of vocal fatigue early may be one of the key issues in the prevention of developing vocal pathology. Due to its common prevalence, it is pertinent to identify and assess VF and provide appropriate treatment.
      Various outcome measures are used to assess and quantify VF including perceptual quality rating, acoustic measures, aerodynamic measures, video stroboscopic examination, and voice range profiles.
      • Yiu EML
      • Chan RMM
      Effect of hydration and vocal rest on the vocal fatigue in amateur karaoke singers.
      Outcomes have been variable and mixed across different studies assessing VF.
      • Hunter EJ
      • Titze IR
      Quantifying vocal fatigue recovery: Dynamic vocal recovery trajectories after a vocal loading exercise.
      ,
      • Lei Z
      • Fasanella L
      • Martignetti L
      • et al.
      Investigation of vocal fatigue using a dose-based vocal loading task.
      Additionally, since the concept of VF is a collection of self-reported symptoms, there is a need for a self-report tool to evaluate and detect it.
      • Welham NV
      • Maclagan MA
      Vocal fatigue: current knowledge and future directions.
      ,
      • Solomon NP
      Vocal fatigue and its relation to vocal hyperfunction.
      Such a tool, the Vocal Fatigue Index (VFI) was developed by Nanjundeswaran et al. (2015).
      • Nanjundeswaran C
      • Jacobson BH
      • Gartner-Schmidt J
      • et al.
      Vocal Fatigue Index (VFI): development and validation.
      This tool is a 19-item questionnaire that consists of three parts designed to measure three factors: “tiredness of voice and avoidance of voice use” (Factor #1 with 11 items), “physical discomfort” (Factor #2 with 5 items), and “improvement of symptoms with rest (Factor #3 with 3 items). The items on the VFI are graded by choosing one of five possible answers: never (0), almost never (1), sometimes (2), almost always (3), and always (4), and a higher sum on factors 1 and 2 and a lower score on factor one indicates that a participant presents with pronounced VF.
      To compare clinical data obtained from different populations and make meaningful recommendations, it is necessary to study and determine whether the measurement tools are reliable and valid for the target society with standardized procedures.
      • Lohr KN
      Assessing health status and quality-of-life instruments: attributes and review criteria.
      Using standardized and validated research instruments enables comparisons of results across different studies both nationally and internationally and increases the certainty with which the instruments accurately reflect what they are supposed to measure.
      • Gjersing L
      • Caplehorn JR
      • Clausen T
      Cross-cultural adaptation of research instruments: Language, setting, time and statistical considerations.
      Up to this moment, the VFI has been translated into several other languages, including Persian,
      • Naderifar E
      • Moradi N
      • Farzadi F
      • et al.
      Cross-Cultural Adaptation and Validation of the Vocal Fatigue Index into Persian.
      Malayalam,
      • Athira UR
      • Devadas U
      Adaptation and Validation of Vocal Fatigue Index (VFI) to Malayalam Language.
      Spanish,
      • Contreras-Regatero S
      • Vila-Rovira J
      • Verdejo C
      Validity and reliability of spanish version of two questionnaires of vocal fatigue in female teachers.
      ,
      • Cantor-Cutiva LC
      • Malebrán Bezerra de Mello MC
      • Hunter EJ
      Translation and adaptation of the Vocal Fatigue Index into Spanish.
      Brazilian Portuguese,
      • Zambon F
      • Moreti F
      • Ribeiro VV
      • et al.
      Vocal Fatigue Index: Validation and Cut-off Values of the Brazilian Version.
      Turkish,
      • Şırın S
      • Öğüt MF
      • Bılgen C
      Reliability and validity of the Turkish version of the Vocal Fatigue Index.
      German,
      • Barsties v. Latoszek B
      • Auner M
      • Graf S
      Cross-Cultural Adaption and Validation of the Vocal Fatigue Index in German.
      Hong Kong-Chinese,
      • Kwong E
      • Tsang MLC
      Cross-Cultural Adaptation and Validation of the Hong Kong-Chinese Version of Vocal Fatigue Index (VFI(HK)).
      Croatian
      • Bonetti A
      • Bonetti L
      • Čipčić O
      Self-assessment of vocal fatigue in muscle tension dysphonia and vocal nodules: a preliminary analysis of the discriminatory potential of the croatian adaptation of the vocal fatigue index (VFI-C).
      and Tamil,
      • Srinivas S
      • Mahalingam S
      • Boominathan P
      Cultural and linguistic adaptation of vocal fatigue index for tamil-speaking population (VFI-T).
      All the published versions of the VFI showed good psychometric properties and may be applied to evaluate the presence, degree, and progression of vocal fatigue in the respective populations.
      There is no such scale in the Polish language to appropriately capture and measure the construct of VF. Thus, the present study aimed at culturally and linguistically adapting and validating the VFI for the Polish population to screen individuals experiencing VF symptoms.

      METHODS

      Step 1: developing the Polish VFI

      Approval for this study was granted by the Ethical Committee of the Medical University of Lodz (decision no. RNN/96/20/KE).

      Methods

      Following the guidelines by Lohr et al.,
      • Lohr KN
      Assessing health status and quality-of-life instruments: attributes and review criteria.
      the forward-backward translation was implemented to linguistically and culturally translate the original version of VFI
      • Nanjundeswaran C
      • Jacobson BH
      • Gartner-Schmidt J
      • et al.
      Vocal Fatigue Index (VFI): development and validation.
      into Polish.
      The forward translation was performed by the first author (JM) of the current study, (i.e., the informed translation) and, independently, by a translator blind to the purpose of the study, (i.e., the uninformed translation). Following this, the research team discussed the discrepancies between the two forward translations and combined them into a synthesized Polish version. Subsequently, two native English speakers independently completed the back translation of this version. A couple of back-translated items (#5 and #10) were slightly different. Our research team combined these two versions into a single version, selecting the items of each translation that, in our opinion, most accurately captured the meaning of the original items. The consensus version of the back-translated Polish VFI was reviewed by CN (author of the original VFI) for semantic, conceptual, and experiential equivalence assessment and suggested reconsidering four items: 9 - It is effortful to produce my voice after a period of voice use, 10 - I find it difficult to project my voice with voice use, 13 - I experience throat pain at the end of the day with voice use, 15 - My throat aches with voice use. After discussing the above-mentioned items among our research team, we concluded that the differences between the original and translated items were subtle and mainly reflected differences between linguistic contexts. Therefore, no further adaptation of the translated items was considered necessary at that point.

      Pilot study

      The Polish pre-test alpha version of the VFI was then administered to 15 participants to assess comprehensibility, readability, and typographical accuracy in patients from the target setting to minimize misunderstanding and subsequent measurement errors. The participants were patients who sought help at the Department of Otolaryngology, Head and Neck Oncology of the Medical University of Lodz due to voice problems. Each person completed the questionnaire and was interviewed for content and response for each item. Both the meaning of the items and responses were discussed by the first author (JM) with each completing participant. The steps of the translation protocol are shown in Figure 1. There were no changes made to the questions resulting in the Beta version of the Polish VFI.
      FIGURE 1
      FIGURE 1A flowchart of the VFI translation protocol.

      Step 2: validation and reliability

      Participants

      Informed consent was sought and confidentiality was ensured by a numerical cross-referencing system. Data collection took place from May 2021 till February 2022.
      A total of 323 subjects participated in the study and completed the Beta version of the Polish VFI (Appendix 1). The study group (1) consisted of 216 patients aged 51.04 ± 11.02 [mean±SD], 143 women, and 63 men who sought clinical services at the Department of Otolaryngology, Head and Neck Oncology of the Medical University of Lodz due to voice problems. Specific inclusion criteria included ages between 18 and 65 years, presenting with vocal complaints, and confirmatory laryngological diagnosis. The patients enrolled in the study were diagnosed with a range of voice disorders, both functional and organic as identified using an otolaryngological examination and laryngovideostroboscopy (LVS) performed using a transnasal flexible video-endoscope with LED light (model CV-170 HD, ENF-VH, Olympus Corp, Tokyo, Japan) by an experienced otorhinolaryngologist. See Table 1 for the medical diagnosis of the study group. The exclusionary criteria were: no record of any surgical procedures in the throat area before the study, no surgery with endotracheal intubation in the six months before the study, and no symptoms of upper respiratory tract infection reported at the time of examination. Participants over 65 years of age were deliberately excluded from the study. Certain voice changes are a normal part of the aging process and voice disorders tend to occur in the elderly as a result of natural anatomical and physiological changes.
      • de Araújo Pernambuco L
      • Espelt A
      • Balata PMM
      • et al.
      Prevalence of voice disorders in the elderly: a systematic review of population-based studies.
      To eliminate the potential influence of the aging process on the quality of voice, patients over the age of sixty-five were not considered suitable for this study.
      TABLE 1A Medical Diagnosis of Study Group patients (N = 216) Based on Laryngological Examination with LVS
      Medical diagnosisNumber of cases (%)
      Total216 (100%)
       Hyperfunctional dysphonia51 (23.6%)
       Vocal fold nodules20 (9.2%)
       Glottal insufficiency29 (13.4%)
       Vocal fold paralysis14 (6.5%)
       Polyp10 (4.6%)
       Reinke's oedema10 (4.6%)
       Vocal Fold Leukoplakia20 (9.2%)
       Chronic laryngitis13 (6.01%)
       Cancer49 (22.7%)
      The Control Group (0) were 107 healthy individuals - normophonic subjects aged 48.49 ± 13.35 [mean±SD] years, 63 women and 44 men who reported no laryngological symptoms, no vocal complaints, no history of laryngeal disorders, and no signs of laryngeal abnormality in LVS.
      Regarding the sample size, an ideal ratio of respondents to items recommended is 10:1, ie at least 10 participants for each scale item.
      • Boateng GO
      • Neilands TB
      • Frongillo EA
      • et al.
      Best practices for developing and validating scales for health, social, and behavioral research: a primer.
      This study meets the suggested criterion for an appropriate sample size. We utilized convenience sampling, where all patients who reported to the clinic and outpatient clinic and met the inclusion criteria were included.
      Additional eligibility criteria for both the study and control groups included obtaining Maximum Phonation Time (MPT), an objective measure quantifying the aerodynamic efficiency of the glottis.
      • Speyer R
      • Bogaardt HCA
      • Passos VL
      • et al.
      Maximum phonation time: variability and reliability.
      MPT is the longest period during which a patient can sustain phonation of a vowel /a:/ on one deep breath at a comfortable pitch and loudness. The calculated average of three measurements was considered MPT.
      The study group (1) and control group (0) were similar in their mean age. Maximum Phonation Time measurement revealed statistically significant differences between the Study Group (1) and Control Group (0) where mean MPT was 7.97s (min 2s, max 18s, SD 2.95) and 14.01s (min 9s, max 28s, SD 2.87s) for these groups respectively.

      Methods

      The Beta version of the Polish version of the VFI was administered to all participants of the study: study group (1) and control group (0). In the original version of the VFI, the scores of the three factors are typically interpreted separately due to the negative wordings used in the items of parts 1 and 2 and, contrastively, the positive wordings used in the items of part 3.
      • Nanjundeswaran C
      • Jacobson BH
      • Gartner-Schmidt J
      • et al.
      Vocal Fatigue Index (VFI): development and validation.
      These parts of the VFI differ conceptually and each of them provides particular and detailed insight into the individual experience of vocal fatigue. For the Polish VFI, to obtain a reliable total score, a sum of individual factors was calculated with the inverted value of Factor 3. One hundred and four participants from the Study Group (1) completed the VFI a second time within a 7–10-day interval. This time interval was considered short enough to avoid substantial voice changes and at the same time long enough for the participants to not be able to recall their previous responses. In the case of patients referred to voice therapy (N=41), the completion of the VFI questionnaire for the second time took place right before the start of the first session.

      Statistical analysis

      Statistical analysis was performed using IBM SPSS Statistics version 20. The results were considered statistically significant if the p-value was less than 0.05 (P < 0.05). The internal consistency of the VFI test and its subscales was calculated using Cronbach's alpha coefficient. Individual items’ contribution to the reliability index was assessed. Values equal to or higher than 0.7 were considered good reliability. Intraclass Correlation Coefficient (ICC) estimates and their 95% confidence intervals were calculated based on a mean-rating, absolute-agreement, 2-way mixed-effects model. The analysis was performed on the 104 subjects who repeated the test, and ICC was calculated between the two test administrations. A component was considered to demonstrate a floor or ceiling effect if more than 15% of participants scored the minimum (floor) or the maximum (ceiling) possible score leading to implications on the questionnaire's reproducibility and responsiveness.
      • Terwee CB
      • Bot SDM
      • de Boer MR
      • et al.
      Quality criteria were proposed for measurement properties of health status questionnaires.
      To deal with the parametric and nonparametric variables of the study, Spearman rank correlation (rho) was performed to investigate the correlation between VFI individual subscale scores. Similarly, Spearman rank correlations were determined between VFI scores and the scores of the two voice self-assessment questionnaires used as reference tools: VHI and V-RQOL. The mean values of the VFI study group (1) and control group (0) were compared using an independent t test. Subsequently, VFI results were compared among diagnoses-based subgroups of patients from the study group using two-way ANOVA. Levene Test for Equality of Variances was conducted to determine variance equality between groups. Depending on the Levene test results, assumptions regarding variance equality were made accordingly.

      Internal consistency

      The measurement of the internal consistency showed a very high Cronbach's alpha coefficient for the total VFI score: α = 0.979. Each factor of the VFI showed very good internal consistency with Cronbach's alpha value of 0.971 for Factor 1, 0.929 for Factor 2, and 0.930 for both Factor 3 and Factor 3 Inv. A detailed analysis of each item of the test (Table 2) indicates that the Cronbach's alpha value did not increase after eliminating any item, thus all the items have a comparable influence on the reliability of the whole scale.
      TABLE 2Internal Consistency (Cronbach's alpha) of the VFI – Item Analyses Results
      Item #Cronbach's alpha if item deleted
      1.978
      2.977
      3.977
      4.977
      5.977
      6.977
      7.979
      8.977
      9.977
      10.977
      11.977
      12.977
      13.977
      14.978
      15.977
      16.978
      17.932
      17 Inv.978
      18.932
      18 Inv.978
      19.935
      19 Inv.979

      Test-retest reliability

      Intraclass Correlation Coefficient (ICC) determined for the reproducibility of the test was satisfactory for each VFI Factor, with each item meeting the set ICC cut-off of >0.70. (Table 3)
      TABLE 3Test-Retest Reliability (Intraclass Correlation [ICC]) of the Polish VFI
      ICCConfidence Interval 95%
      lower bandupper band
      VFI Total
      Calculated with the inverted value of Factor 3.
      0.8590.7980.902
      VFI Factor 10.8290.7580.881
      VFI Factor 20.8320.7620.883
      VFI Factor 3 / VFI Factor 3 Inv0.7290.6250.808
      Detailed response distribution values (percentage) for each VFI item for Time Point 1 and Time Point 2 are included in the Supplementary Table.
      low asterisk Calculated with the inverted value of Factor 3.

      Floor & ceiling effects

      Floor and ceiling effects occur when a considerable proportion of subjects score the worst/minimum or best/maximum score, rendering the measure unable to discriminate between subjects at either extreme of the scale.
      • Stucki G
      • Liang MH
      • Stucki S
      • et al.
      Application of statistical graphics to facilitate selection of health status measures for clinical practice and evaluative research.
      In this study, there was no floor and ceiling effect for the VFI test score in the Study Group (1) and the Control Group (0) suggesting that it can be used as a valid patient-reported outcome measure.

      Criterion and construct validity

      Assessing the correlation between the individual subscales of the VFI scale, a satisfactory statistical significance was found for the VFI Total score and its all subscales, as well as between all the subscales of the test (Table 4), with the strongest correlation between VFI Total and VFI Factor 1 (r = .980, P < 0.001).
      TABLE 4Analysis of the Construct Validity of the Polish VFI. All Correlations Were Significant With P < 0.01
      VFI Total
      Calculated with the inverted value of Factor 3.
      VFI Factor 1VFI Factor 2VFI Factor 3VFI Factor 3 Inv
      VFI Total
      Calculated with the inverted value of Factor 3.
      1.980.946−.849.849
      VFI Factor 1.9801.899−.764.764
      VFI Factor 2.946.8991−.783.783
      VFI Factor 3−.849−.764−.7831−1.000
      Calculated with the inverted value of Factor 3.
      VFI Factor 3 Inv.849.764.783
      Calculated with the inverted value of Factor 3.
      −1.0001
      low asterisk Calculated with the inverted value of Factor 3.
      Voice Handicap Index (VHI) and Voice-Related Quality of Life (V-RQOL) measures were used in this study as reference tools for criterion validity, as they are validated self-assessment tools that have proved reliable in several studies on the Polish population
      • Niebudek-Bogusz E
      • Kuzańska A
      • Woznicka E
      • et al.
      Assessment of the voice handicap index as a screening tool in dysphonic patients.
      • Sinkiewicz A
      • Garstecka A
      • Mackiewicz-Nartowicz H
      • et al.
      The effectiveness of rehabilitation of occupational voice disorders in a health resort hospital environment.
      • Sielska-Badurek E
      • Rzepakowska A
      • Sobol M
      • et al.
      Adaptation and validation of the voice-related quality of life measure into polish.
      and have been commonly used in clinical practice.
      VHI is a patient self-assessment test first introduced by Jacobson et al. in 1998.
      • Jacobson B
      • Johnson A
      • Grywalski C
      • et al.
      The Voice Handicap Index (VHI): development and validation.
      It consists of 30 items in three domains: emotional, physical, and functional aspects and it quantifies the amount of disability that a voice disorder is causing. Points from the questions, from 0 to 4, can be combined to assign a total score (maximum 120 points), or they can be combined by subscale. The higher the number, the greater the amount of disability noted due to a voice-related problem. The V-RQOL, developed by Hogikyan and Sethuraman in 1999
      • Hogikyan ND
      • Sethuraman G
      Validation of an instrument to measure voice-related quality of life (V- RQOL).
      consists of 10 questions in total on physical and mental functioning in the past two weeks. A sum score ranging from 10 to 50 is obtained and subsequently calculated from zero to 100, using the algorithm suggested by the authors of the questionnaire. The higher V-RQOL score indicates a better overall voice-related quality of life. The Study Group and Control Group participants answered the three questionnaires (VFI, VHI, V-RQOL) during the initial evaluation session, before the laryngological examination.
      The correlations between VFI and two reference questionnaires, VHI and V-RQOL are presented in Table 5. A statistically significant correlation was found between the total score of VFI and VHI (r = .915, P < 0.001). The correlation was significant also between individual VFI factors and VHI domains, with the strongest correlation between VFI Factor 1 (Tiredness of voice and voice avoidance) and the VHI Emotional domain (r = .877, P < 0.001). There was a significant correlation between the total VFI score and the total V-RQOL score (r = -.834, P < 0.001). The correlation is negative, meaning that with the increased vocal fatigue symptoms the perceived voice-related quality of life decreases.
      TABLE 5Correlations between VFI and Two VHI and V-RQOL Scores. All correlations were significant with P < 0.01.
      VHI TotalVHI FunctionalVHI EmotionalVHI PhysicalVRQOL Total
      VFI Total
      Calculated with the inverted value of Factor 3.
      .915.877.885.852−.834
      VFI Factor 1.907.866.877.848−.790
      VFI Factor 2.884.852.854.825−.814
      VFI Factor 3−.760−.738−.734−.717.776
      VFI Factor 3 Inv.760.738.734.717−.776
      low asterisk Calculated with the inverted value of Factor 3.

      Discriminant validity

      The VFI mean scores in the Study Group and Control Group differ significantly (in t test: P < 0.001). The mean VFI Total scores in the Study Group were significantly higher than that in the Control Group (0) with 48.88±11.73 [mean±SD] points vs. 12.25±6.75 [mean±SD] points respectively. Similar statistically significant differences were observed for all three factors of the test. The statistical findings for the Total and Factor scores of VFI are presented in Table 6. Figure 2 shows VFI Total score frequency distribution in Control Group (0) and Study Group (1).
      TABLE 6Mean Total and Factor VFI results for Study Group (N = 216) and Control Group (N = 107)
      GroupMeanSDMinMaxSignificance
      VFI TOTAL
      Calculated with the inverted value of Factor 3.
      Control12.257.73340P<0.001
      Study48.8811.731672
      VFI Factor 1Control6.634.93622P<0.001
      Study28.447.581044
      VFI Factor 2Control2.022.62010P<0.001
      Study10.983.19318
      VFI Factor 3Control11.391.15713P<0.001
      Study5.542.84015
      VFI Factor 3 Inv.Control3.611.1528P<0.001
      Study9.462.84015
      Abbreviations: SD, standard deviation; VFI, vocal fatigue index.
      low asterisk Calculated with the inverted value of Factor 3.
      FIGURE 2
      FIGURE 2VFI Total score frequency distribution in Control Group (N = 107) and Study Group (N = 216).
      The mean Total VFI scores were compared in diagnosis-based subgroups of patients in the Study Group (Table 6). The highest scores, indicating greater vocal fatigue, were observed in the Cancer group with M  =  55.27; SD 5.077, followed by Reinke's edema group with M  =  49.7; SD 12.48, and vocal fold paralysis group with M  =  45.43; SD 8.23. See Table 7 for the distribution of VFI scores across different diagnoses.
      TABLE 7Mean Total VFI results in diagnose-based subgroups of the Study Group (1)
      Diagnosis (N)MeanSDSEM95% Confidence IntervalMinMaxSignificance
      Lower bandUpper band
      Hyperfunctional dysphonia (N=51)40.456.540.91538.6142.292755P<0.001
      Vocal fold nodules (N=20)40.46.431.4337.3943.412849P<0.001
      Glottal insufficiency (N=29)42.387.11.3139.6845.082566P<0.001
      Vocal fold paralysis (N=14)45.438.232.2040.6750.182657P<0.001
      Polyp (N=10)36.911.653.6828.5745.231962P<0.001
      Reinke's oedema (N=10)49.712.483.9540.7758.633576P<0.001
      Leukoplakia (N=20)44.27.241.6240.8147.593161P<0.001
      Chronic laryngitis (N=13)39.927.021.9535.6844.172853P<0.001
      Cancer (N=49)55.275.070.7253.8156.723164P<0.001

      Sensitivity and specificity

      Methods

      An ROC curve was constructed to measure the test's ability to correctly classify subjects with and without vocal fatigue. The area under the ROC curve (AUC) classifies the level of accuracy of a diagnostic test.
      • Behlau M
      • Madazio G
      • Moreti F
      • et al.
      Efficiency and cutoff values of self-assessment instruments on the impact of a voice problem.
      The cut-off point of the VFI was determined to discriminate between normophonic subjects and vocal fatigue subjects (the results were confirmed by using a classification tree). Additionally, all three Factors of the VFI questionnaire were submitted to an ROC curve analysis to determine the discriminating power of each factor.

      Results

      The best cut-off giving the best relationship between sensitivity and specificity for VFI Total Score was set at 32.5 score. This score, with sensitivity of 0.931 and specificity of 0.991 shows the test's best ability to discriminate between subjects with and without vocal fatigue. The analysis of the determined AUC (Table 8) revealed diagnostic accuracy of the VFI for all factors and the total score: AUC > 0.962. Figure 3 shows ROC curves for the particular Factors of the VFI. The best discriminating power (the highest value of AUC) is observed for the Total score: AUC  =  0.993. The area under the curve is also significantly higher than 0.7 for all three factors, indicating that the items in the VFI can differentiate between healthy individuals and those with vocal fatigue.
      TABLE 8Diagnostic Accuracy Results of the Polish VFI
      VFIAROCThresholdSensitivitySpecificity
      VFI Total
      Calculated with the inverted value of Factor 3.
      0.99332.50.9310.991
      Factor #10.99110.50.9950.785
      Factor #20.9754.50.9910.882
      Factor#3 Inv0.9626.50.8190.981
      low asterisk Calculated with the inverted value of Factor 3.
      FIGURE 3
      FIGURE 3ROC curve analysis for VFI Total and VFI individual Factors: Factor 1, Factor 2, and Factor 3 Inv to discriminate between subjects with vocal fatigue and vocally healthy subjects.

      DISCUSSION

      Vocal fatigue, being a common phenomenon seen in almost every voice disorder, remains the least understood symptom of a voice pathology.
      • D'Haeseleer E
      • Behlau M
      • Bruneel L
      • et al.
      Factors involved in vocal fatigue: a pilot study.
      It is related to increased vocal effort and has the potential of leading to vocal hyperfunction and benign lesions such as vocal fold nodules.
      • McCabe DJ
      • Titze IR
      Chant therapy for treating vocal fatigue among public school teachers: a preliminary study.
      The detection of VF is not straightforward as there are no agreed-upon markers of VF during laryngeal endoscopic exams.
      • Gao Y
      • Dietrich M
      • Desouza GN
      Classification of vocal fatigue using semg: data imbalance, normalization, and the role of vocal fatigue index scores.
      Symptoms can occur despite a normal-appearing larynx and a normal-sounding voice, therefore the diagnosis of vocal fatigue relies on the patient's self-reported symptoms of the experience of this condition
      • Solomon NP
      Vocal fatigue and its relation to vocal hyperfunction.
      Therefore, a standardized questionnaire is particularly suitable for quantifying the extent of activity and participation limitations caused by the effects of vocal fatigue in daily life.
      • Barsties v. Latoszek B
      • Göllner M
      • Mathmann P
      • et al.
      The german restructured vocal fatigue index and characteristics of dysphonic and vocally healthy populations.
      Given that there is no tool to quantify vocal fatigue available for the Polish population, the present study aimed to adapt and validate VFI in the Polish language.
      Overall, all 19 items were retained in the Polish version of the VFI after following the appropriate phases of cultural and linguistic adaptation guidelines by the International Quality of Life Assessment. The factors from the original version were considered a priori and maintained in the Polish version of the VFI.
      • Acquadro C
      • Conway K
      • Hareendran A
      • et al.
      Literature review of methods to translate health-related quality of life questionnaires for use in multinational clinical trials.
      A voice disorder or the lack thereof was confirmed based on an expert-based diagnosis made during a formal ENT examination (with LVS) and measurement of MPT. This method is in line with most other VFI validation studies in which a wide range of clinically diagnosed voice disorders was reported, for instance in studies by Naderifar et al.,
      • Naderifar E
      • Moradi N
      • Farzadi F
      • et al.
      Cross-Cultural Adaptation and Validation of the Vocal Fatigue Index into Persian.
      Barsties v. Latoszek et al.,
      • Barsties v. Latoszek B
      • Auner M
      • Graf S
      Cross-Cultural Adaption and Validation of the Vocal Fatigue Index in German.
      Zambon et al.
      • Zambon F
      • Moreti F
      • Ribeiro VV
      • et al.
      Vocal Fatigue Index: Validation and Cut-off Values of the Brazilian Version.
      Bonetti et al.,
      • Bonetti A
      • Bonetti L
      • Čipčić O
      Self-assessment of vocal fatigue in muscle tension dysphonia and vocal nodules: a preliminary analysis of the discriminatory potential of the croatian adaptation of the vocal fatigue index (VFI-C).
      Cantor-Cutiva et al.,
      • Cantor-Cutiva LC
      • Malebrán Bezerra de Mello MC
      • Hunter EJ
      Translation and adaptation of the Vocal Fatigue Index into Spanish.
      although in the last study mentioned the sample size was comparatively smaller than in other studies (N  =  34).
      The Polish VFI showed excellent internal consistency for the total score calculated with the inverted value of Factor 3. The results are consistent with other versions of the VFI.
      • Contreras-Regatero S
      • Vila-Rovira J
      • Verdejo C
      Validity and reliability of spanish version of two questionnaires of vocal fatigue in female teachers.
      ,
      • Athira UR
      • Devadas U
      Adaptation and Validation of Vocal Fatigue Index (VFI) to Malayalam Language.
      ,
      • Naderifar E
      • Moradi N
      • Farzadi F
      • et al.
      Cross-Cultural Adaptation and Validation of the Vocal Fatigue Index into Persian.
      ,
      • Cantor-Cutiva LC
      • Malebrán Bezerra de Mello MC
      • Hunter EJ
      Translation and adaptation of the Vocal Fatigue Index into Spanish.
      ,
      • Şırın S
      • Öğüt MF
      • Bılgen C
      Reliability and validity of the Turkish version of the Vocal Fatigue Index.
      • Barsties v. Latoszek B
      • Auner M
      • Graf S
      Cross-Cultural Adaption and Validation of the Vocal Fatigue Index in German.
      • Kwong E
      • Tsang MLC
      Cross-Cultural Adaptation and Validation of the Hong Kong-Chinese Version of Vocal Fatigue Index (VFI(HK)).
      ,
      • Srinivas S
      • Mahalingam S
      • Boominathan P
      Cultural and linguistic adaptation of vocal fatigue index for tamil-speaking population (VFI-T).
      Data from test-retest reliability for the Polish version of the VFI on each of the three factors confirmed satisfactory reliability of the VFI during the time interval. These results were consistent with other studies pertaining to the linguistic and cultural adaptations.
      • Athira UR
      • Devadas U
      Adaptation and Validation of Vocal Fatigue Index (VFI) to Malayalam Language.
      ,
      • Naderifar E
      • Moradi N
      • Farzadi F
      • et al.
      Cross-Cultural Adaptation and Validation of the Vocal Fatigue Index into Persian.
      ,
      • Şırın S
      • Öğüt MF
      • Bılgen C
      Reliability and validity of the Turkish version of the Vocal Fatigue Index.
      • Barsties v. Latoszek B
      • Auner M
      • Graf S
      Cross-Cultural Adaption and Validation of the Vocal Fatigue Index in German.
      • Kwong E
      • Tsang MLC
      Cross-Cultural Adaptation and Validation of the Hong Kong-Chinese Version of Vocal Fatigue Index (VFI(HK)).
      Results pertaining to no floor and ceiling effect can be regarded as a positive finding since it denotes practical significance in terms of the appropriate formulation of the items and the range of possible responses.
      The significant differences between the group with voice disorders and the control group for the total VFI score and all three factors suggest that the VFI can effectively identify the vocal fatigue symptoms in subjects with voice disorders. This finding is in line with the findings of the original study, where individuals with voice disorders had significantly higher mean scores for all three VFI Factors compared to healthy individuals without voice disorders. Interestingly, we observed a difference between our results and those in the original study concerning the mean scores for Factor #3. High scores reported by the control group with voice disorders in this part of the questionnaire might suggest that in the case of vocally healthy individuals, even if/when some VF symptoms are experienced, they easily subside with vocal rest. The subjects with voice disorders, however, report little improvement of symptoms with voice rest. One possible explanation for this could be the fact that our study group comprised a substantial number of severe voice pathology (the Cancer group N  =  49). In the study by Mc Cabe et al.,
      • McCabe DJ
      • Titze IR
      Chant therapy for treating vocal fatigue among public school teachers: a preliminary study.
      it was hypothesized that at some point in the progression of fatiguing a threshold could be reached at which normal soft tissue regeneration might not occur. This threshold could vary from individual to individual, depending on age, and also on the condition of the voice. The severity of the voice disorder could thus increase the recovery time or in some cases, vocal rest could have little or no effect on the perceived severity of symptoms. An additional change in our study was to invert the Factor 3 scores in order to arrive at a total VFI score. Such practice is not uncommon and has been utilized for calculating the total score of the Aging Voice Index developed by Etter et al.
      • Etter NM
      • Hapner ER
      • Barkmeier-Kraemer JM
      • et al.
      Aging voice index (AVI): reliability and validity of a voice quality of life scale for older adults.
      Even though each factor of VFI is conceptually different the total VFI score may be beneficial for quantifying the overall severity of VF symptoms.
      In investigating construct validity, the main purpose is to determine if the scale can measure the specific concept it is designed to measure. To validate the Polish VFI against currently used valid and reliable voice scales, participants were asked to complete the VHI and V-RQOL questionnaires. The correlation between VFI and V-RQOL is inverse, indicating that the greater vocal fatigue reported by an individual results in a decreased voice-related quality of life. Both total scores and individual factors showed strong correlations, with the correlation between VFI Total and VHI Total being the strongest (r = -.915; P < 0.001). Strong correlations between VFI and VHI might indicate that vocal fatigue is perceived by the affected individuals as a handicap, limiting their functional, physical and emotional wellbeing. The weakest correlations were observed for VFI Factor #3 and all subscales of VHI. In a study by Afkhami et al. investigating the correlation between VFI and VHI scores in persons with laryngeal pathologies, the correlation of the third factor of VFI with factors of VHI was not significant.
      • Afkhami M
      • Amirishavaki Y
      • Qhelichi L
      • et al.
      Correlation between vocal fatigue index and voice handicap index scores in Persons with laryngeal pathologies.
      In the original study by Nanjundeswaran et al. the discriminative ability of the VFI was reported as excellent (AUC: 0.91, 90% sensitivity, and 90% specificity for each factor; cut-off values for factors 1, 2, and 3 were ≥24, ≥7, and ≤7, respectively)
      • Nanjundeswaran C
      • Jacobson BH
      • Gartner-Schmidt J
      • et al.
      Vocal Fatigue Index (VFI): development and validation.
      . In our study, the threshold for the VFI Total was set at a 32.5 score (AUC  =  0.993, 0.931 sensitivity, 0.991 specificity). The cut-off points for the individual factors were ≥ 10.5 (AUC  =  0.991, 0.995 sensitivity, 0.785 specificity) for Factor 1, ≥ 4.5 (AUC  =  0.975, 0.991 sensitivity, 0.882 specificity) for Factor 2, ≥ 6.5 (AUC  =  0.962, 0.819 sensitivity, 0.981 specificity) for Factor 3 Inv. Again, inverting the value of Factor 3 and calculating the total score proves beneficial, as the Total score shows the highest value of AUC, which is the best discriminating power. Both our research and other studies on validating the VFI in different languages show differences in thresholds established for particular factors as compared to those presented by Nanjundeswaran et al.
      • Athira UR
      • Devadas U
      Adaptation and Validation of Vocal Fatigue Index (VFI) to Malayalam Language.
      ,
      • Barsties v. Latoszek B
      • Auner M
      • Graf S
      Cross-Cultural Adaption and Validation of the Vocal Fatigue Index in German.
      ,
      • Kwong E
      • Tsang MLC
      Cross-Cultural Adaptation and Validation of the Hong Kong-Chinese Version of Vocal Fatigue Index (VFI(HK)).
      It has been concluded by Kwong and Tsang that among-version discrepancies in cut-off scores may be attributed to the differences in culture in the corresponding populations.
      • Kwong E
      • Tsang MLC
      Cross-Cultural Adaptation and Validation of the Hong Kong-Chinese Version of Vocal Fatigue Index (VFI(HK)).
      An interesting observation in the present study is the fact that based on the threshold value in Table 8 for the VFI total (32.5), it seems that some of the participants in the Control Group (0) would be described as having vocal fatigue – the maximum VFI total score for this group was 40. There are numerous reasons that could explain the fact that this score lies within the score range for the dysphonic population. Firstly, vocal production can be disadvantaged by several factors, not necessarily resulting from laryngeal pathology. First, a philosophy of “the mind-body connection” has been documented in the literature. It encompasses the paradigm that psychosocial factors impact the body and cause somatic symptoms.
      • Ziadni MS
      • Carty JN
      • Doherty HK
      • et al.
      A life-stress, emotional awareness, and expression interview for primary care patients with medically unexplained symptoms: a randomized controlled trial.
      Another way in which psychosocial factors can affect the voice is by causing physiological changes within the muscles of the body when emotions are heightened. Muscular tension within the larynx can cause limited vocal fold flexibility and increased effort to produce and sustain the voice.
      • López JM
      • Catena A
      • Montes A
      • et al.
      Effectiveness of a short voice training program for teachers: a preliminary study.
      Finally, the VFI is a subjective measure and we have to bear in mind the fact that some individuals may be more sensitive to the changes in their voices than others, for instance, occupational voice users who rely on their voices for work. The degree of vocal fatigue in different occupational groups has not been investigated in this study.
      The present research comprises a variety of laryngeal pathologies, as we were interested to see if the perceived vocal fatigue severity is in line with the clinical-instrumental evaluation of laryngeal status. The results demonstrated that the highest VFI scores indicating greater vocal fatigue were observed in patients with organic voice disorders: laryngeal cancer, Reinke's edema, and vocal fold paralysis. However, these findings should be interpreted with caution at this point. Future studies should include a comparable number of subjects in all laryngeal pathology subgroups. It would be interesting to investigate the severity of vocal fatigue symptoms in groups of voice pathologies of organic and functional origin. Also, the comparison of vocal fatigue symptom severity could be explored in different occupational groups of voice users. To date, several such studies have been conducted. For instance, Moghtader et al. investigated vocal fatigue in University professors,
      • Moghtader M
      • Soltani M
      • Mehravar M
      • et al.
      The relationship between vocal fatigue index and voice handicap index in university professors with and without voice complaint.
      Anand et al. focused on prospective vocal professionals,
      • Anand S
      • Bottalico P
      • Gray C
      Vocal fatigue in prospective vocal professionals.
      Dodderi, Johnson and Aji measured vocal fatigue in beat boxers
      • Dodderi T
      • Johnson A
      • Aji AM
      Vocal fatigue in beat boxers.
      and Erkan et al. examined factors potentially contributing to vocal fatigue in doctors.
      • Erkan SO
      • Tuhanioglu B
      Vocal fatigue in doctors: evaluation with subjective and objective acoustic parameters.
      Other groups of interest with high vocal demand could include call center workers, tour guides, shop assistants, or fitness instructors.

      CONCLUSIONS

      The Polish version of VFI is a reliable and valid measurement tool that can be used for identifying, quantifying, and evaluating vocal fatigue symptoms in the Polish-speaking population. As a validated tool, it can be effectively used in both clinical and research settings.

      Acknowledgments

      We are immensely grateful to Chayadevie Nanjundeswaran who provided insight and expertise that greatly assisted the research.

      Appendix B. SUPPLEMENTARY DATA

      Appendix 1: Polish version of Vocal Fatigue Index

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